The Mesoproterozoic Beaverhead Impact Structure and Its Tectonic Setting, Montana‐Idaho: 40Ar/39Ar and U‐Pb Isotopic Constraints

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Abstract

New 40Ar/39Ar and uranium‐lead (U‐Pb) zircon data from the Beaverhead impact structure, first identified by extensive shatter coning of Proterozoic quartzite and gneiss from the Beaverhead Mountains near the Montana‐Idaho border, indicate that the structure formed at or after 900 Ma. The 40Ar/39Ar age spectra from fine‐grained muscovite and biotite from a breccia zone in high‐grade gneiss show significant argon loss but yield dates for highest‐temperature steps that cluster between 899 and 908 Ma. The dated minerals probably formed by recrystallization of impact glass, so on both geologic and isotopic grounds, the dates probably represent the minimum age of impact. U‐Pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of $$2464\pm 56$$ Ma and a lower intercept apparent age of $$779\pm 69$$ Ma. Another brecciated gneiss about 7 km to the northeast that does not contain secondary mica does contain zircons that yield a concordant apparent age of $$2455\pm 9$$ Ma. Nearby gneiss that neither is brecciated nor contains shatter cones yields an apparent age of $$2451\pm 46$$ Ma. The 40Ar/39Ar results constrain the age of the shatter‐coned quartzite and indicate that it is >900 Ma and possibly correlative with the Gunsight Formation of the Mesoproterozoic Lemhi Group. The upper intercept U‐Pb age of ∼2450 Ma from all three dated samples also shows that the Paleoproterozoic basement rocks of the area are among the youngest in the mostly Archean Wyoming province of North America. The impact site lies near the margin of the province, along the northeast‐trending Great Falls tectonic zone, and the relatively young crustal age may reflect Early Proterozoic marginal accretion.

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